The som package

A Kohonen Self-organising Map (SOM) maps input patterns
onto a regular grid (usually two-dimensional) where each
node in the grid is a model of the input data, and does
so using a method which ensures that any topological
relationships within the input data are also represented
in the grid. This implementation supports the use of
non-numeric patterns.

In layman's terms, a SOM can be useful when you you want
to discover the underlying structure of some data.